Known silver halides include silver iodide, silver bromide, silver chloride, silver iodochloride, silver chlorobromide, silver iodochlorobromide, etc. A variety of shapes of the silver halide crystals particles (grains) are also known. So-called regularly shaped crystal forms include cubic, octahedral, tetradecahedral, rhombic dodecahedral or the like. Spherical, tabular, amorphous or the like are examples of irregularly shaped crystalline particles. Further, multiphase structural crystal particles having layered structures or conjugate (joined) structures in the particles are also in common use. The halogen composition, shape and structure of these crystal particles are known to influence the characteristics and properties of the silver halide particles, as noted, for example, by T. H. James in The Theory of the Photographic Process (4th Ed., Macmillan Co., Ltd., New York) (particularly, the description in the first and third chapters of the properties of silver halides, and the description in the third chapter of the shapes of silver halides, etc.).
Silver halide emulsions may exhibit various characteristics, depending upon the halogen composition of the particles used therein. For example, a silver chloride emulsion has a low sensitivity but has a high solubility and, therefore, is suitable for rapid processing as such an emulsion is capable of undergoing high speed development and fixation. However, fog often occurs in silver chloride emulsions. On the other hand, when a silver bromide emulsion is used, development processing is somewhat slower, but fog hardly occurs, and, further, the light sensitivity of this type of emulsion is high. Silver iodide emulsions are extremely difficult to develop, and, therefore, are rarely used alone in photographic materials. However, mixed silver halide crystals comprising silver iodide and silver bromide exhibit an excellent light sensitivity and, therefore, silver halide emulsions containing such a mixture of crystal particles are extremely important in photographic light-sensitive materials used as camera films.
A variety of techniques have heretofore been known, utilizing the characteristics of various kinds of these silver halides, and there is a substantial amount of literature publications concerning core-shell layered structures of silver halide particles. Typically, the entire surface of the core is coated with one or more shells having a silver halide composition which is different from that of the core. Japanese Patent Publication No. 18939/81 teaches that a silver halide emulsion comprising silver bromide (core) and silver chloride (shell) particles combines the high light sensitivity of the silver bromide and the rapid developability of the silver chloride, but these properties of the two types of silver halide become somewhat suppressed in a mixed crystal type silver chlorobromide emulsion. In addition, German Patent Application (OLS) No. 3,229,999 illustrates that core-shell silver halide particles formed from a silver halide layer having at least 25 mol% silver chloride content and a silver halide layer having a smaller silver chloride content (mol%) than the former, the latter being adjacent to the former, are characterized in that the amount of fog formation is small and the pressure property is good.
U.S. Pat. No. 4,094,684 illustrates an emulsion containing silver halide particles formed by epitaxial growth of silver chloride over polyhedral silver iodide crystal particles. Further, U.S. Pat. No. 4,463,087 illustrates an emulsion containing silver salt particles formed by epitaxial growth of (111) surface-surrounded and silver iodide-containing host silver halide particles and a method for the preparation thereof; and U.S. Pat. No. 4,471,050 illustrates an emulsion comprising silver halide host particles having a face-centered cubic type crystalline structure and non-isomorphous silver salts as projecting only from the edges or corners of the host particles. Furthermore, Japanese Patent Publication No. 24772/83 (corresponding to U.S. Pat. No. 4,496,652) describes cubic silver halide crystals where the corner parts have a different halogen composition from that of the center body part of the crystal, illustrating that it is possible for such crystals to have a selectivity to the introduction of impurities thereinto and to control the crystal defects thereof.
The silver halide particles having this type of structure (as described in Japanese Patent Publication No. 24772/83) are also described by C. Hasse, H. Frieser and E. Klein in Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden, Vol. 2 (Akademische Verlagsgesellschaft, Frankfurt an Main, 1968), in which it is stated that the deposition of silver chloride on octahedral silver bromide crystals resulted in the formation of many (100) surface-containing small silver chloride particles on the eight (111) surfaces of the octahedral crystals and that these small particles were attached to the octahedral crystals after the successive deposition of the silver chloride over the crystals to finally form crystalline surfaces of cubic crystals.
According to C.R. Berry and D.C. Skillman in Journal of Applied Physics, 35, 7, 2165 (1964), the deposition of silver chloride on octahedral silver bromide particles also causes the epitaxial formation of silver chlorobromide mixed crystals over the (111) surface of the particles, while the deposition of silver chloride on cubic silver bromide particles causes epitaxial growth or projections only at the corners or edges of the cubic crystals.
In the same manner, C.R. Berry mentions in Photographic Science and Engineering, 19, 3, 29 (1975) that the deposition of silver chloride on dodecahedral particles having both (111) and (100) surfaces preferentially occurs on the (111) surface most often, whereas deposition on the (110) surface occurs next most often, while deposition on the (100) surface hardly occurs. Further, this publication describes that the deposition of silver chloride occurs more readily on the six tetrasymmetric corners than on the other eight tri-symmetric corners among the two kinds of corners present on dodecahedral particles.
In all of these above-described known techniques and publications concerning silver chlorobromide particles, epitaxial growth selectively occurs on the edges or corners of each crystal, or growth occurs on the (111) surface and (110) surface of the crystal. In the aforesaid core-shell type particles, uniform growth causes the covering (the shell) of all the surfaces of the core particles. Under these circumstances, epitaxial conjugate (joined) particles having a silver halide part selectively conjugated and formed on the (100) surface of the core silver halide particles are not known to exist.
On the other hand, with respect to (110) surface-surrounded rhombic dodecahedral particles, German Pat. No. 2,222,297 (corresponding to U.S. Pat. No. 3,817,756) describes silver chloride and silver chlorobromide particles and Japanese Patent Application (OPI) No. 222842/85 describes silver bromide and silver iodobromide particles (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, the particles obtained by these known methods are rhombic dodecahedral particles themselves having twelve (110) surfaces or polyhedral particles which further have six (100) surfaces or eight (111) surfaces introduced into the dodecahedral particles. Such particles are also described in the above-mentioned Photographic Science and Engineering, 19, 3, 29 (1975). However, the shapes of the particles described therein are not defined with particularity, and further, such particles are difficult to obtain by the method described therein. Japanese Patent Application (OPI) No. 83531/86 illustrates silver bromide and silver iodobromide particles which have a groove in the center of the (110) surface. These particles, however, are not conjugate (joined) type particles.
In any event, these known particles described above are rhombic dodecahedral shaped or similarly shaped crystalline particles, and, therefore, the specifically shaped conjugate type particles composed mainly of (110) crystalline surfaces of the present invention are novel.
Development of silver halide particles having a higher sensitivity with less fog formation is a keenly desired goal in the photographic technical field. However, silver halide particles which satisfactorily achieve this goal had not yet been discovered until the present invention described in detail hereinbelow.